This invention relates to a dynamic protection integrated device, in particular for integrated circuits with input stage in MOS technology.
As is known, the input stages of some integrated circuits made in the MOS technology formed of fairly delicate structures require to be protected against electrostatic discharges. To that end, protections are utilized which are typically formed by a diode which intervenes when the input voltage reaches its breakdown voltage, e.g. 15 V. That diode is constructionally fairly strong and capable of protecting the gate and possibly also the diffused zones of the MOS transistors forming the input stage of the integrated circuit to be protected. In general, moreover, on the input there is applied a resistor for protection of the diode itself. In fact, a high tension discharge with very high breakaway current having a certain decay time can already cause the circuit to become damaged.
In FIG. 1 there is shown an implementation where an integrated circuit 3, to the input terminals 2 whereof a voltage is applied, for example mains voltage, is protected by an outer resistor 1 disposed between one of the terminals 2 and the input of the integrated circuit 3. FIG. 1a presents the input of an integrated circuit with MOS input stage having a protection formed by a diode 4 and the resistor 1. Here the input stage is represented schematically by the MOS transistor 5 to the gate of which the input signal is applied through the protection resistor 1, which gate is protected through the diode 4.
This known protection system is adequate when the device is utilized for the typical purpose for which it is designed with an expected input voltage range, but not in the instance where the device must be utilized for a wider voltage range than the designed one. In fact, such devices are utilized for instance with very high voltages, e.g. to detect zero passing, or to control the presence of the mains voltage, or to monitor other electric signals applied to the input, so as to control devices placed downstream. In that case on the input there may be applied a much higher voltage than the typical logic level one (in some cases up to tens or hundreds Volts) with a reduction resistor. In that case there may occur, however, anomalous phenomena which partly hinder or at least disturb the device operation.
For example, high positive potentials cause the oxide of the MOS gate to become damaged with consequent destruction of the input stage, or may cause breakdown of the protection diode, whereas negative voltages bring about a forward bias current through the diode. For example, in the instance of application of an alternating voltage during the positive halfwave, the input voltage may go up to the value of intervention of the protection. There occurs then a sharp descent of the voltage to the lower intervention value and again climbing back with a series of oscillations about the lower cut off value. That value is close to the high logic value and involves therefore noise in the instance of logic operation of the MOS integrated circuit. In the instance of the negative halfwave, there occurs an even worse behavior. In fact, as soon as the diode begins to conduct forward it injects electrons into the substrate, discharging the floating nodes (where preloaded data are present, for example) within a certain distance from the input terminal. Consequently, the device works no more and the results obtained are unreliable. That effect becomes more significant at less high frequencies and as the number of the MOS structures in the IC, connected to the input stage, increases.